The present invention relates to an internal combustion engine for a vehicle, more particularly to a cylindrical trunk piston for use in the engine.
Generally, in internal combustion engines for a vehicle such as automobiles, pistons are made of an aluminum alloy, one of the light alloy metals, to reduce the weight thereof, thereby reducing the vibration of the engine.
In a conventional internal combustion engine, a trunk type piston 10 as shown in FIGS. 1 and 2 comprises a piston crown section 12, a pair of piston pin boss sections 14, a piston skirt section 6, etc. integrally formed in an aluminum alloy casting. Some examples of such pistons are described in "AUTOMOBILE ENGINEERING HANDBOOK--new edition--", 1970, by Automobile Technology Institute, pages 4 to 20.
An aluminum alloy having a small specific gravity is used to reduce the weight of the piston 10 as described above. In addition, the excellent heat conductive property of the aluminum alloy is effective to reduce the temperature of the piston crown section 12 (particularly, the piston head section which forms a part of the combustion chamber). Thus, there is an advantage that the knocking-resistant property is improved.
However, since the coefficient of thermal expansion of the aluminum alloy is larger than that of iron, as the temperature of the piston 10 changes depending upon the driving condition of the engine, the dimension of the piston 10 also changes. This results in a drawback that the sliding gap between the piston 10 and the cylinder (not shown) largely changes. More specifically, if the piston 10 is formed such that its diameter is smaller than a predetermined value to provide a gap enough to accomodate the thermal expansion of the piston during operation at high temperatures, the gap will become very large when the temperature of the piston 10 is low. Consequently, the piston slap (side pressure sound) will increase.
Accordingly, the piston 10 in the prior art has the following construction to keep a desired constant sliding gap between the piston 10 and the cylinder.
The piston skirt section 16 serves to guide the piston 10 sliding in the cylinder. This piston skirt section 16 has a generally oval cross section. As the temperature rises, the piston crown section, exposed to the relatively higher temperature in the combustion chamber, thermally expands, which is followed by the expansion of the thick piston pin boss section 14, so that the piston skirt section 16 pulled by the boss section 14 is deformed. The thin piston skirt section 16 itself also expands due to heat. The deformation of the piston skirt section pulled by the boss section and the thermal expansion of the piston skirt section are well balanced by design in order to keep the above-mentioned sliding gap constant.
The temperature in the piston skirt section 16 generally decreases longitudinally along the axis of the piston from the upper portion 18 to the lower portion 22 (refer to FIG. 2) during driving of the autmobile. Therefore, the piston skirt section 16 is tapered to keep the expansion of the piston skirt section uniform; more specifically, the lateral cross section of the upper portion 18 of the piston skirt section 16 is smaller than that of the lower portion 22 so that the difference in thermal deformation between the upper and lower portions 18 and 22 is suitably adjusted on the basis of the difference in temperature of the both portions.
Incidentally, the term lateral is intended to indicate the direction orthogonal to the longitudinal axis of the piston 10.
The conventional piston construction as described above has a drawback that since the temperatures in the upper and lower portions 18 and 22 of the piston skirt section 16 vary largely depending upon the driving state, the thermal expansion and deformation also change during operation, and therefore it is impossible to keep the gap between the cylinder and the upper portion 18 constant throughout the driving of the automobile.
While the piston is operated under a relatively low load, in other words, when the piston temperature is relatively low, the piston skirt section 16 does not deform enough and keeps substantially its tapered shape, so that the gap between the cylinder and the upper portion 18 of the piston skirt section 16 is left large. Hence, the piston 10 vibrates or swings in the cylinder, which results in the increased piston slap. The improper reciprocating operation of the piston ring causes the oil consumption and the amount of blow-by gas to increase.
A primary object of the present invention is therefore to provide a trunk piston for use in an internal combustion engine in which the gap between the cylinder and the piston can be kept small, so that the piston slap is reduced.
Another object of the present invention is to provide a trunk piston for use in an internal combustion engine in which the oil consumption and the amount of blow-by gas are reduced.